Field of the Invention
The present invention relates to a method of producing a lightguide plate which is a component of a surface light source used for a back light of a liquid crystal device or the like, a lighting advertisement, a traffic-control sign, etc., and a lightguide plate for surface light source and a surface light source using the same.
As shown in FIG. 10, a surface light source 201 serving as a back light is arranged on the side of the rear surface of a liquid crystal panel (LCD) 200. The surface light source 201 comprises a lightguide plate 202 composed of a transparent flat plate, fluorescent tubes 203 arranged on both its side surfaces, and a lens film 204 arranged on a light emitting surface 202a of the lightguide plate 202.
A surface on the upper side in the drawing of the lightguide plate 202 is the above-mentioned light emitting surface 202a, a surface on the lower side to be the reverse side of the light emitting surface 202a is a light transmission preventing surface 202b, and a side surface of the lightguide plate 202 is a light incident surface 202c. When light from the fluorescent tubes 203 is incident on the light incident surface 202c, the light is totally reflected in the lightguide plate 202, and is all propagated to the whole area of the lightguide plate 202. A part of the propagated light becomes diffused reflected light at less than a critical angle by light scattering reaction of the light transmission preventing surface 202b, and is emitted from the light emitting surface 202a of the lightguide plate 202. The emitted light leads to the lens film 204.
The lens film 204 has a projection of a triangular prism-type lenticular lens on its one surface, and the other surface thereof is a smooth surface. The lens film 204 is affixed to the surface 202a of the lightguide plate 202 with the projection directed upward. The light emitted from the surface 202a of the lightguide plate 202 is isotropically and uniformly diffused in a predetermined angular range by passing through the lens film 204 (see Japanese Utility Model Laying-open Gazette No. 107201/1992).
Furthermore, Japanese Utility Model Laying-Open Gazette No. 162201/1980 discloses a lightguide plate for surface light source in which a light scattering layer constructed by mixing particles having a light scattering effect and adhesives is formed between a transparent plastic plate and a reflecting plate.
Furthermore, U.S. Pat. No. 4,729,067 discloses a technique using a mat transparent diffusing plate (a mat transparent sheet) in place of the above-mentioned lens film 204. However, a technique using the lens film 204 or a technique simultaneously using the lens film 204 and the mat transparent diffusing plate is more advantageous in that light energy of a light source is intensively distributed in a desired restricted angular range, and diffused light high in uniformity and isotropy is obtained in the angular range, as compared with the technique using the mat transparent diffusing plate.
Furthermore, a surface light source 210 shown in FIG. 11 is known. The surface light source 210 comprises a plate-shaped lightguide plate 212, a fluorescent tube 213 arranged on its one side surface, a diffusing plate 214 arranged on a surface 212a on the light emission side of the lightguide plate 212, and first and second lenticular lens sheets 215a and 215b arranged on a surface on the light emission side of the diffusing plate 214. The fluorescent tube 213 comprises a cold-cathode tube 213a and a reflector 213b. Each of the first and second lenticular lens sheets 215a and 215b is constructed by forming a lot of lens portions in a prism shape on its one surface. The lenticular lens sheets 215a and 215b are overlapped with each other upon being so arranged that the directions in which the lens portions in a prism shape are formed are perpendicular to each other. The surface light source 210 having such a structure has the advantage that high luminance is achieved. On the other hand, the surface light source 210 comprises the two lenticular lens sheets 215a and 215b, whereby the cost thereof is increased.
A surface light source given a lens function by forming a concavo-convex pattern in a material itself composing a lightguide plate has been also known.
Description is now made of a method of producing a lightguide plate for surface light source constructed by forming a concavo-convex pattern in a material itself composing the lightguide plate. In producing a lightguide plate for surface light source, a molding die comprising a pair of metal molds which can be divided is used. It is possible to obtain a lightguide plate for surface light source by processing a portion constituting a cavity in the pair of metal molds into a shape corresponding to the size and the outer shape (the plane shape, the concavo-convex shape of the surface, etc.) of a required lightguide plate, coupling the metal molds to each other to form the cavity, injecting resin liquid into the cavity, solidifying its resin liquid and then releasing the resin from the cavity.
In the conventional method of producing a lightguide plate, however, a concavo-convex shape corresponding to a recessed and projected surface of the lightguide plate is formed in the metal mold itself, whereby the cost of the molding die is increased, and the cost of the molding die is reflected in the cost of the lightguide plate, whereby the cost of the lightguide plate is increased. When a plurality of types of lightguide plates which differ in the size and the outer shape are formed, a molding die is required for each type, whereby the cost thereof is further increased.
Japanese Patent Laying-Open Gazette No. 117144/1995 discloses a technique in which the necessity of forming a concavo-convex shape in a metal mold itself is eliminated. This technique is a method of obtaining a lightguide plate 301 having a recessed and projected surface using a material itself composing the lightguide plate by injecting resin into a cavity in a state where a resin sheet 300 having a concavo-convex shape is arranged in the cavity, solidifying the resin, and then stripping the resin sheet 300, as shown in FIGS. 12A, 12B and 12C.
In a method using the resin sheet 300, however, it is difficult to repeatedly use the resin sheet 300 many times. As more lightguide plates 301 are produced, therefore, more resin sheets 300 are consumed. Consequently, the cost thereof is increased by the consumption, and the number of industrial wastes is increased by discarding the resin sheets 300.
The present invention has been made in view of the foregoing circumstances, and has for its object to provide a method of producing a lightguide plate for surface light source having a concavo-convex shape without forming a concavo-convex shape in a metal mold itself and without using a resin sheet. The present invention also has for its object to provide a method of producing a lightguide plate for surface light source in which it is possible to form a light transmission preventing surface (a reflecting surface) composed of a thin film simultaneously with the resin injecting step. The present invention also has for its object to provide a lightguide plate for surface light source capable of achieving higher luminance without providing two lenticular lens sheets, and a surface light source comprising the lightguide plate for surface light source, as contrasted with the prior art shown in FIG. 11.